The present invention relates to a process for phosphating a metal surface to make thereon a zinc phosphate film for coating use. More particularly, it relates to a process for phosphating the surface of a metallic matter to make thereon a zinc phosphate film suitable for electrodeposition coating, particularly for cationic electrodeposition coating, excellent in adhesion and corrosion-resistance, particularly in resistance for warm brine and scab corrosion (hereinafter, the term "resistance for scab corrosion" is referred to as "anti-scab property"), wherein the metal surface is intend to mean an iron-based surface, a zinc-based surface, an aluminum-based surface, or a metal surface having two or more kinds of these surfaces together and simultaneously, in particular, a metal surface having an aluminum-based surface, which comprises a part processed with an abrasive, and an iron-based and/or zinc-based surfaces together and simultaneously.
There have been used metallic materials in various kinds of articles such as car bodies and other automobile parts, building materials, and furniture, etc. The metallic materials are processed as pre-treatment to make a zinc phosphate coating film in order to avoid corrosion due to oxygen and sulfur oxides in the atmosphere and to rain and sea water. The zinc phosphate coating film thus-formed is desired to be excellent in adhesion with a metal surface, that is a substrate, and with a film being thereon formed and also, desired to have sufficient rust-resistance under the corrosive environment. Especially, because the car bodies are repeatedly exposed to brine and a change of dry and wet weather conditions through scratches of the outer plate parts, anti-scab property and high order of resistance for warm brine, etc. are strongly desired. In the present invention, the term "a phosphating process" used herein is employed to mean "a process for phosphating a metal surface to make thereon a zinc phosphate coating film".
Recently, there are increasing cases where metallic materials having two or more kinds of metal surfaces are phosphated with zinc phosphate to make a phosphate film. For example, in order to further elevate the corrosion-resistance of car bodies, there has been employed a material which is plated with zinc or alloyed zinc at only one side of steel material. If a hitherto-known zinc phosphating process is carried out for a metal surface, as mentioned above, which has an iron-based and a zinc-based surfaces together and simultaneously, there takes place a problem that the corrosion-resistance and secondary adhesion of a zinc-based surface are inferior compared to those of an iron-based surface. Because of this, for example, there has been proposed, in Japanese Official Patent Provisional Publication, showa 57-152472 etc., a process for making a zinc phosphate film suitable for electrodeposition coating on a metal surface having an iron-based and a zinc-based surfaces together and simultaneously. In a phosphating bath of this process, wherein concentrations of a zinc and phosphate ions as well as that of an accelerator for forming a coating film with conversion are controlled, a manganese and/or nickel ions are contained in concentrations of 0.6 to 3 g/l and/or 0.1 to 4 g/l, respectively. Also, there is proposed, in Japanese Official Patent Gazette, showa 61-36588, an art in which 0.05 g/l or more of a fluoride ion are added together with a manganese ion in order to lower a processing temperature.
Moreover, a material made of combining an aluminum material with an iron material or a zinc material has been practically used in various kinds of articles such as automobiles and building materials. If a material of this kind is processed with an acidic solution for making a zinc phosphate film which has so-far been employed for an iron or zinc material, an aluminum ion dissolving into the phosphating solution accumulates and, when the accumulated amount becomes higher more than a certain extent, a problem of converting inferiority on an iron-based surface takes place. That is, if the aluminum ion becomes 5 ppm or more in a phosphating solution which does not contain a fluoride ion, 100 ppm or more in a phosphating solution which contains HBF.sub.4, or 300 ppm or more in a phosphating solution which contains H.sub.2 SiF.sub.6, converting inferiority occurrence has been found on an iron-based surface.
Thus, to prevent an increase of the aluminum ion in a phosphating solution, there has been proposed, in Japanese Official Patent Provisional Publication, showa 57-70281, a process which comprises precipitating the aluminum ion as K.sub.2 NaAlF.sub.6 or Na.sub.3 AlF.sub.6 by adding potassium acid fluoride and sodium acid fluoride into a phosphating solution. Also, there has been proposed, in Japanese Official Patent Provisional Publication, showa 61-104089, a process which comprises controlling a proportion in area of an aluminum-based surface to an iron-based surface in 3/7 or less and maintaining concentration of the aluminum ion in 70 ppm or less.
The zinc phosphating process disclosed in the Japanese Official Patent Provisional Publication, showa 61-104089, has a disadvantage by which a matter for making a coating film with a phosphating process (hereinafter, simply referred to as "phosphating object") is very limited and, in addition, it is difficult to maintain the aluminum ion concentration at 70 ppm or less by means of only controlling the forementioned area proportion. On the other hand, the phosphating process disclosed in the Japanese Official Patent Provisional Publication, showa 57-70281, is superior in points of that the processing objects is not limited and an idea of removing the aluminum ion in a phosphating solution with precipitating has been adopted. However, a precipitate formed herein shows a tendency of floating with suspending and makes non-uniform a zinc phosphate coating film by attaching to it. Because of this, in a case where electrodeposition coating is carried out on a zinc phosphate coating film, inferior electrodeposition coating takes place and, as a result, it becomes a factor for lack of coating film uniformity and inferior secondary adhesion in the coating film. Accordingly, there is a necessity to remove the floating and suspending precipitate, but this removing works is very complicate.
The present inventors undertook researches to solve the problems in previous arts as described above and, as a result, invented a process, in which a simple fluoride is added into a phosphating solution taken out from a phosphating bath in order to remove the aluminum ion with precipitating and then, the solution is again returned to the phosphating bath and, as a result, the aluminum ion concentration in the bath is maintained at a definite value or less, and which was applied for a patent, Japanese Patent Application, heisei 2-36432. According to this process, because the aluminum ion concentration is always maintained within a proper range, inferior conversion on a metal surface does not take place. Besides, since any precipitate is not formed in a phosphating bath, any bad influence by the precipitate upon a coating film does not take place.
However, even by a phosphating process in the forementioned previous arts, in a case where a part or a whole of the aluminum-based metal surface has been processed with an abrasive, it was found that in this part processed with an abrasive any zinc phosphate coating film is not formed or a non-uniform coating film is only formed, so that there is a problem by that corrosion-resistance in the part becomes very inferior. This is, in an aluminum-based metal, because an inactive film is formed on a surface by being processed with an abrasive and, by this inactive film, formation of a coating film is disturbed.
Even in the previous arts, if the active fluorine concentration in a phosphating solution is enhanced, the converting is improved by removing with dissolving the inactive film in the part processed with an abrasive, but when the active fluorine concentration is high, an amount of the dissolving aluminum ion increases in a part other than the part processed with an abrasive, that is an abrasive-nonprocessed part, and thus, an aluminum ion precipitation in the phosphating bath occurs in a great extent, a concentration of sludge floating and suspending in a phosphating solution in a phosphating bath, that is the precipitate concentration, becomes high and, as a result, there takes place inferior electrodeposition coating by attaching of the precipitate to a processing object.